Outline-2, BIO 3220, Muscular System


A. Overall function – Locomotion, posture, protection, heat production

B. Classification

1. Skeletal (is striated)

a. Muscle fiber – Muscle cell

1. Multinucleate

2. Myofibrils – Striated cylinders within myofiber

3. Myofilaments – Proteins which interlock and result in contraction

a. Actin – Thin myofilament; has receptor site for myosin

b. Myosin – Thick myofilament; has receptor site for actin & ATP

c. Contraction- Myosin heads attach to actin and with the energy from ATP do a Power Stroke, swiveling towards the center of the sarcomere resulting in shortening of sarcomere.

4. Sarcomere – Distance from Z line to Z line; basic unit of contraction; Actin is attached to Z line & is pulled towards the center of the sarcomere by myosin’s power stroke.

b. Motor neurons & motor units – Skeletal muscle cannot contract without stimulation from a motor neuron. The motor neuron plus the number of myofibers it innervates comprise a motor unit.

c. Somatic muscles {all skeletal M. are in this category except branchiomeric M.(which are skeletal muscle but visceral muscles since they operate the visceral skeleton}

1. Voluntary

2. Body wall & appendages

a. Tail

b. Hypobranchial

c. Tongue

d. Extrinsic eyeball

3. Myotome derivatives [segmented] (phylogenetically or by ontogeny) or Hypomere (Refer back to early development notes.)

4. Orients body in external environment

d. Color & Endurance

1. Red fibers

a. More blood supply – aerobic metabolism

b. Myoglobin – Red pigmented protein that stores oxygen

c. Fatigue resistant

d. Fish-cruising long distances; tetrapods-posture

2. White fibers

a. Anaerobic metabolism

b. Fatiguable

c. Fish-spurts of swimming; tetrapods-sprints

2. Cardiac Muscle

a. Intercalated disks – Important in linking all cardiac muscle cells together so that the heart contracts in unison.

b. Involuntary

c. Lateral plate mesoderm in origin

3. Smooth muscle

a. Involuntary

b. Lateral plate mesoderm origin

c. Regulates internal environment

d. Innervated by ANS

e. Smooth muscle is in wall of tubular & hollow organs = visceral

f. Intrinsic eye muscles

g. Erectors of feathers & hairs

C. Gross features of skeletal muscle

1. Origin, insertion

2. Tendon, aponeurosis, fascia

3. Muscle shapes

a. Strap [sternocleidomastoid]

b. Teardrop [pectorals]

c. Spindle [extremities]

d. Heads

e. Sheets [abdominals]

f. Fan [trapezius]

g. Circular/sphincter

h. Pinnate [diaphragm]

D. Actions of skeletal muscle

1. Flex/extend

2. Adduct/abduct

3. Levator/depressor

4. Protract/retract

5. Constrict/dilator

6. Rotator

7. Supinator/pronator

8. Tensor [taut]

9. AGONIST, ANTAGONIST, SYNERGIST Primary mover; muscle opposing movement; muscle helping movement

E. Development/Phylogeny

1. Position

2. Embryology – Helps to provide for major muscle categories.

3. Nerve supply – Shown to be most consistent indicator of homology.

4. Mesoderm (Refer back to your early development notes.)

a. Dorsal = epimere = Somite

1. Myotome – Becomes most muscles except branchiomeric.

2. Sclerotome & dermatome

b. Lateral plate = hypomere

1. Somatic – Muscles of body wall

2. Splanchnic – Smooth muscle of gut & cardiac muscle


1. Trunk, tail, hypobranchial, tongue, extrinsic eye muscles

2. General features

a. Metamerism – myomeres

1. Develop embryologically from Somites, specifically myotomes

2. Myosepta

3. Hypomere contributes in higher vertebrates


a. Simple

b. Segments

c. Myotomes


a. Horizontal / Lateral Septum

b. Epaxial Muscles

1. Myotomes

2. Dorsal rami of spinal nerves [tetrapods too]

3. Extend spine & some lateral flexion

4. Extrinsic eye muscles

a. Head myotomes

b. Cranial Nerves III, IV, VI

5. Epibranchial

c. Hypaxial Muscles

1. Myotomes

2. Ventral rami of spinal nerves

3. Ventroflex, lateral bending

4. Hypobranchial

a. Location

b. Origin

c. Function – Respiratory and feeding

5. TETRAPODS – SEE Vertebrate muscles page

a. Epaxial trunk muscles – Dorsal, skull to tail

1. Dorsalis trunci {amphibians, not Anurans}

2. Intervertebrals – Deep

3. Longissimus – Lateral & long; dominant spine extensor

4. Spinales – Medial spine muscle

Multifidus spinae – Combination of Spinales & Intervertebral in lumbar region in some mammals to stabilize lumbar region.

5. Iliocostalis – Most lateral epaxial spine muscle. Important in reptile undulating motion.

6. Trends

Decreased in tail & trunk in reptiles, birds, mammals, but increased in neck

b. Hypaxial – Trend for myomeres to disappear and sheets of muscles form a sling

1. Subvertebrals – Below transverse processes; Flex spine

2. Lateral – Support & compress body wall

a. Obliques

b. Transverse

c. Intercostals – Amniotes only

3. Rectus abdominis – Ventroflex & compress abdomen

4. Diaphragm – Breathing muscle unique to mammals

c. Hypobranchial and Tongue muscles

1. Function – Stabilize hyoid & larynx

2. Tongue [Lingu-,Gloss-]

d. Extrinsic eye muscles

1. Voluntary

2. Obliques – Rotates eye along its transverse axis

3. Rectus – Up, down, side to side

4. Retractor in some


1. Definition – Innervated by ventral ramus of spinal nerves


a. Origin – Extensions of hypaxials of body wall

1. Paired fins – From myotome

2. Median dorsal fin – Not appendicular muscles/From myotome of epaxials

3. Median ventral fin – Not appendicular muscles/From myotome of hypaxials

b. Dorsal mass of fins – extensors/abductors

c. Ventral mass – flexors/adductors

3. TETRAPOD– SEE Vertebrate Muscle page

a. Pectoral limb

1. Dorsal group

a. Deltoid – Arm abductor

b. Latissimus dorsi – Very constant among vertebrates; limb retractor

c. Teres major – Medial rotator of arm; formed from Latissimus dorsi

d. Subcoracoscapularis>>> subscapularis – Scapula to humerus

e. Scapulohumeralis anterior>>> teres minor – Rotates humerus

f. Triceps – Humerus to ulna; Forearm extensor

g. Forearm & hand extensors – Lateral epicondyle of humerus to wrist/hand

h. Cutaneous trunci = Cutaneous maximus – Armpit to trunk; twitches skin

2. Ventral group

a. Pectoralis – Humerus adductor; Important wing muscle to lower wing

b. Supracoracoideus – Important wing muscle to elevate wing

1. Supraspinatus – Extends humerus

2. Infraspinatus – Humerus lateral rotation

c. Coracobrachialis – Arm flexor/adductor

d. Biceps brachii & Brachialis – Forearm flexor

e. Forearm, wrist & digit flexors – Medial epicondyle of humerus to wrist & digits

b. Pelvic limb

1. Dorsal group

a. Iliofemoralis & Iliofibularis – Thigh abductors

1. Gluteus

2. Tensor fasciae latae

b. Iliotibialis; femorotibialis, Ambiens – Extend tibia, protract thigh

1. Quadriceps

2. Sartorius – From Ambiens

c. Puboischiofemoralis Internus – Limb protractor

1. Iliacus

2. Psoas major

3. Pectineus

d. Ankle & Foot Extensors – Course laterally

2. Ventral Group

a. Puboischiotibialis – Thigh retractor; knee flexor

1. Gracilis

b. Pubotibialis & Flexor tibialis – Thigh retractor; knee flexor

1. Semimembranosus – Hamstring

2. Semitendinosus – Hamstring

3. Biceps femoris – Hamstring

4. Adductor longus

c. Adductor femoris – Reptiles & mammals

d. Ankle & Foot flexors – Course medially


1. Visceral origin; from hypomere

2. Striated & Voluntary

3. First Arch

a. Adductor mandibulae – Closes jaw

1. Temporal

2. Masseter

{3. Pterygoid}

b. Intermandibularis – Raises floor of mouth

1. Mylohyoid

2. Digastric (anterior)

3. Tensor tympani

4. Second Arch

a. Levator hyomandibulae – Raises upper jaw in hyostylic suspension

1. Stapedius

b. Dorsal hyoid constrictor = Levator hyoideus – Constricts pharyngeal cavity

1. Stylohyoid

c. Interhyoideus {ventral hyoid constrictor}

1. Depressor mandibulae – Important muscle in all vertebrates but mammals

2. Digastric (posterior)

3. Sphincter colli/Platysma in mammals

5. Third Arch

a. Constrictors – Dorsal & Ventral; Compresses pharynx

b. Levators – Lifts gill bars

c. Adductors – Deep; Closes internal angles of gill arch

d. Interarcuals – Expands pharynx

e. a-d Stylopharyngeus – Swallowing in Tetrapods

6. Arch 4-6

a. Constrictors

b. Levators

c. Adductors

d. Interarcuals

e. a-d – Breathing & swallowing muscles




f. Cucullaris– Lifts posterior gill

1. Trapezius

2. Sternomastoid

3. Cleidomastoid

See Vertebrate Muscles Page


1. Incidence – Found in more than 500 species

2. Derivation

a. Muscle cells – Primarily from muscle cells

(b. Glandular)

(c. Nervous tissue)

3. Function

a. Communication

b. Orientation / objects

c. Detection of prey

d. Offense/Defense

4. Electroplax – Functional unit of electric organ; modified muscle cell

a. Multinucleate – Richly innervated; numerous mitochondria

b. Column –> Organ – Columns of stacked cells, up to 10,000, comprise the electric organ

c. Electric potentials – Very high is some; Salt water eel can emit 50 Volts; Fresh water eel can emit 500 Volts.